Replicacion

• DNA


  The genetic material
• The discovery of bacterial transformation: the Griffith experiment

  1. Virulent S cells killed by boiling

  2. Heat-killed S cells injected into mice

  3. Mice survive

  4. Mixture of heat-killed S cells and live R cells injected into mice

  5. Mice die

  6. Live S cells recovered from dead mice

  7. Heat-killed S cells transform live R cells into live S cells
• Genes were known to be associated with specific traits, but their physical nature was not understood
• Mutations were known to alter gene function, but the precise chemical nature of a mutation was not understood
• The one-gene–one-enzyme hypothesis postulated that genes determine the structure of proteins
• Genes were known to be carried on chromosomes
• Chromosomes were known to consist of DNA and protein
• Evidence that DNA is the genetic material in bacteria: the Avery, Macleod, and McCarty experiments


  1. Extract of dead S cells destroyed one chemical component at a time

  2. Polysaccharides, lipids, RNAs, proteins destroyed but extract still transformed

  3. DNA destroyed and extract lost transforming ability

  4. DNA is the transforming agent and genetic material
• Evidence that DNA is the genetic material in phage: the Hershey–Chase experiment


  1. Phage T2 structure studied

  2. Phage DNA labeled with 32P, proteins labeled with 35S

  3. 32P recovered in bacteria, 35S recovered in phage ghosts

  4. DNA, not protein, is the genetic material of phages
• Radioisotopes


  Unstable (radioactive) isotopes of an element that emit radiation to transform into a more stable form
• Phosphorus is not found in the amino acid building blocks of proteins but is found in DNA
• Sulfur is not in the nucleotide building blocks of DNA but is in proteins
• Genetic material


  • Must allow accurate replication
  • Must have informational content
  • Must be able to change (mutate) on rare occasion
• Nucleotides


  The fundamental building blocks of DNA, composed of a phosphate group, a deoxyribose sugar, and one of four bases (adenine, guanine, cytosine, thymine)
• Purines


  Adenine and guanine, bases with a double-ring structure
• Pyrimidines


  Cytosine and thymine, bases with a single-ring structure
• La polimeraza sintetiza en sentido 3'--5'
• Deoxyguanosine 5′-monophosphate (dGMP)


  Nucleotide composed of a phosphate, deoxyribose sugar, and guanine base
• Cytosine (C)


  Pyrimidine base found in DNA
• Deoxycytidine 5′-monophosphate (dCMP)


  Nucleotide composed of a phosphate, deoxyribose sugar, and cytosine base
• Deoxythymidine 5′-monophosphate (dTMP)


  Nucleotide composed of a phosphate, deoxyribose sugar, and thymine base
• Thymine (T)


  Pyrimidine base found in DNA
• Nucleotides are the fundamental building blocks of DNA. All nucleotides have a phosphate, a sugar, and a base. The sugar is called deoxyribose because it is a variant of ribose that lacks an oxygen atom. There are two purine bases (adenine and guanine) and two pyrimidine bases (cytosine and thymine).
• The chemical subunits of DNA are nucleotides or, more specifically, deoxynucleotides, each composed of a phosphate group, a deoxyribose sugar molecule, and one of the four bases (Figure 7-5). It is convenient to refer to each nucleotide by the first letter of the name of its base: A, G, C, or T.
• Deoxyadenosine 5′-monophosphate (dAMP)


  Nucleotide composed of a phosphate, deoxyribose sugar, and adenine base
• DNA contains an equal amount of A and T nucleotides and G and C nucleotides. Organisms vary in the relative amount of A + T versus G + C, but different tissues in the same organism have the same relative amount of A + T versus G + C.
• DNA nucleotides are known as deoxynucleotides and are composed of a phosphate, a deoxyribose, and a purine or pyrimidine base.
• The total amount of purine nucleotides (A + G) always equals the total amount of pyrimidine nucleotides (T + C).
• The amount of A always equals the amount of T, and the amount of G always equals the amount of C; that is, A/T and G/C is close to 1.0, regardless of the source of DNA.
• The amount of A + T is not necessarily equal to the amount of G + C, as can be seen in the last column of Table 7-1. The (A + T)/(G + C) ratio varies among different organisms.
• The X-ray diffraction pattern of DNA showed that it is a long and skinny, two-stranded helix (that is, a double helix).
• Pairing of purines with pyrimidines accounts exactly for the diameter of the DNA double helix determined from X-ray data.
• The two strands of DNA contain complementary base pairs—G base pairs with C and A base pairs with T.
• The base-paired strands of DNA are oriented antiparallel to one another—one strand is oriented in the 5′-to-3′ direction and the other strand is oriented in the 3′-to-5′ direction.
• A–T base pairs have two hydrogen bonds, and G–C base pairs have three.
• The geometry of base pairs creates shallow, wide major grooves and narrow, deep minor grooves along the DNA helix; features that are recognized for protein binding.
• DNA structure


  The double helix model fulfills the three requirements for a hereditary substance: 1) Suggests how the genetic material might determine the structure of proteins, 2) Proposes a possible copying mechanism for the genetic material, 3) Mutations are possible by the substitution of one nucleotide for another
• DNA base pairs


  • A-T base pairs have two hydrogen bonds, G-C base pairs have three
  • The geometry of base pairs creates shallow, wide major grooves and narrow, deep minor grooves along the DNA helix; features that are recognized for protein binding
• DNA Replication


  1. Semiconservative

  2. Unwinding the double helix

  3. Each DNA strand acts as a template to direct assembly of complementary bases

  4. Creates two double helices that are identical to the original
• The Meselson-Stahl experiment demonstrated that DNA is copied by semiconservative replication